disable hibernate and see if it keeps doing it. Sounds like a sleep bug of some kind.

To delete this file, you have to use the command prompt to manually turn off hibernation. If you turn off hibernation via Power Options it will not delete the actual file itself.
Open a command prompt in Windows 7 by typing command in the search box. Right click Command Prompt and choose Run as Administrator.
To turn off or disable hibernation, type the following command at the prompt:

powercfg –h off

This will disable hibernation and will remove the hiberfil.sys file from your system.